One common type of retroreflective construction contains small spherical lens elements, such as glass microspheres, and either diffusely or specularly reflecting material adjacent the rear surface thereof, the selection of refractive indices of the microspheres and positioning of the respective components being made in a known manner to provide maximum retroreflective efficiency. In almost all commercial retroreflective constructions having a sheet form the specularly reflective material is a metal, i.e., a vapor-coated layer of specularly reflective aluminum arranged behind the microspheres, either directly on the microsphere, as taught in U.S. Pat. No. 3,005,382 (Weber) or U.S. Pat. No. 3,190,178 (McKenzie), or spaced from the microspheres by a spacing layer as taught in U.S. Pat. No. 2,407,680 (Palmquist et al.). Alternatively, specularly reflecting metal flakes can be contained in the binder, as disclosed in U.S. Pat. No. 2,567,233 (Palmquist et al.).
Unfortunately, metallic specular reflectors have several serious disadvantages, one of which is that the color of the light reflected from a retroreflective construction using such a reflector is not readily controlled, and a brilliant white appearance is difficult to achieve, especially when aluminum is used. Colored effects are generally limited to the peculiar color characteristics of the available specular reflecting materials, unless color is introduced into the microspheres or is contained in a top-coating over the microspheres. Also, aluminum is subject to corrosion.
The limitations on obtaining retroreflective sheeting colored other than gray is a problem especially in reflectorizing fabrics. Reflective and retroreflective sheeting and fabrics have been proposed as a means to provide greater visibility, and therefore improved safety, to pedestrians or cyclists traveling along streets or highways at nighttime. For example, U.S. Pat. No. 2,567,233 (Palmquist et al.) discloses a flexible, weather-resistant sheet comprising a layer of transparent microlenses which are partially embedded in a resilient light reflective binder pigmented with metal flakes. The sheets may be provided with an adhesive on the back side in order to be bonded to various substrates such as garments, fabric, wood, and metal. Such sheets, however, have an unappealing gray color and a brightness of typically about 40 candella per square meter per lux. U.S. Pat. No. 3,172,942 (Berg) discloses a retroreflective transfer film, also gray, which is applied to fabric articles with a hot iron. U.S. Pat. No. Re. 30,892 (Bingham et al.) discloses a method for retroreflective treatment of fabric whereby minute retroreflective particles supported in a softenable binder material are deposited onto the fabric in a sparsely spaced manner. The latter binder material is softened during application to provide adhesion of the particles to the fabric. While these known retroreflective fabric treatments are useful for some applications, they do not generally have as high a brightness as is desired for some uses.
In addition to metals, other specularly reflective materials have been proposed. For instance, U.S. Pat. No 3,700,305 (Bingham) discloses retroreflective construction containing microspheres with adjacent dielectric mirrors. The dielectric mirrors are made up of a contiguous array of materials, at least one being a layer form, having an alternating sequence of refractive indices, all except the material furthermost from the microspheres necessarily being transparent. U.S. Pat. No. 3,758,192 (Bingham) discloses retroreflective constructions containing a monolayer of microspheres which are substantially hemispherically surrounded by a binder containing specularly reflective nacreous pigment particles.